The last dewatering device on a paper machine fourdrinier wire, just before the press section, is normally a series of suction boxes. Depending on the type of paper produced the number of the suction boxes and the pressure at which they are operated will vary. In addition the couch roll and press section pick-up roll also have a vacuum applied and are normally operated at a lower absolute pressure than the suction boxes. The vacuum in these suction boxes is maintained by the use of one or several liquid ring vacuum pumps.
The exhaust from such vacuum pumps is normally vented into the atmosphere and no heat is recovered due to the relatively low quality of heat (low temperature). The air which is being sucked through the wire and the fiber web, both, mechanically pull the water off the moving fibers as well as flash cool the water. This results in the air moving through the vacuum pump essentially being saturated with water vapor. During the compression cycle in the vacuum pump further water is evaporated from the seal water. The same is true for moist air coming from the couch roll and pick-up roll suction boxes.
Another known system utilizes a dry blower vacuum pump and thereby producing a relatively dry hot air. After further heating this air is said to be usable as pocket ventilation air for the paper machine ventilation system. The problem with this known system is the carry-over of fibers or fines from the suction boxes. This fiber will cause problems in the in-line steam heaters, which are normally of the fin type and sensitive to any material suspended in the air stream.